1. Discussion of Prior Art
Various types of automatic electronic flash units have been developed, and basically these units include a flashtube, an electronic power source for the flashtube, a light sensor circuit to measure light from the flashtube which is reflected from the subject being photographed, and a quench circuit activated by the light sensor circuit to turn off the flash after sufficient light has been received at the subject. The sensor circuit includes a light sensor which usually is mounted in the housing of the flash unit, but sometimes is physically detachable and remotely positionable while being connected to the flash unit by a suitable electrical cord. Various such flash units are commercially available, a typical such unit being the Model 283 Electronic Flash Unit sold by Vivitar Corporation.
While such automatic electronic flash units have met with wide acceptance and are used by many professional and amateur photographers, the nature of the light sensor arrangement at or near the flash unit itself for measuring reflected light, rather than measuring light incident on the subject being photographed, results in inaccuracies in precisely controlling the amount of light incident on the subject. Typical flash units of this nature include a sensor which has a finite light sensing angle and are designed to view the subject as being "18% grey." However, if the subject is darker or lighter, or the primary subject is not in the metering center of the scene, the sensor of the flash unit will make an erroneous reading based upon the information supplied in the form of the design of the sensor and sensor system. The sensor of the automatic flash unit depends on the reflectivity of the scene within a defined area specified by the angular view of the light sensor and, thus, close proximity of dominant reflective surfaces also can greatly affect picture quality.
Conventional light meters which measure reflected light also suffer from similar deficiencies. Thus, reflected meter readings are subject to the same deficiencies as are the reflected light readings made by the light sensor of an automatic electronic flash unit. Light meters for measuring "incident" light have been used for years to enable more accurate meter readings under conditions where the subject varies from an average. These incident light meters are positioned near the subject to read the quantity of light impinging thereon, while ignoring the reflectivity of the subject. Even if the reflectivity of the subject changes, if the quantity of light thereon is constant the subject remains correctly illuminated and the incident light meter will so indicate. However, the response characteristics of continuous light measuring types of incident light meters are not sufficient to measure an electronic flash which has a typical duration of less than one to two milliseconds.
Developments have been made in the electronic flash field in an attempt to accomplish more accurate metering by measuring incident light. Such incident light measuring devices provide information by which the user can set his camera aperture for correct flash exposure. Although an incident flash meter can measure a sample output of a flash unit at a given place in the scene and provide good results, there is no assurance that the output of the next flash will be identical. This problem could be solved by placing the light sensor via a remote cable at the subject for metering the incident light during the picture flash exposure; however, the incident light sensor, its support and cable would appear in the photograph and this is undesirable. While this could be overcome by camouflaging or disguising the sensor and/or using a wireless control system back to the flash unit, either approach is bothersome, cumbersome and expensive.
Another attempt which has been made to obviate the foregoing problems is by using a light sensor or sensors which meter the light at the film plane of a camera, such as illustrated in U.S. Pat. No. 3,840,882, U.S. Pat. No. 3,779,141 and U.S. Pat. No. 3,726,197. Such arrangements generally require a special type or "dedicated" electronic flash unit for use with the camera, and also suffer from other metering difficulties such as sensor angle, reflectivity and so forth.
In addition to the foregoing and while not related to the aforementioned problems but related to camera operating characteristics, systems have been proposed for generating a preparatory flash, storing an electrical signal which is a function of the received light reflected from the subject, and then firing a flash or taking the photograph and using the stored signal for quenching the flash. Such a system is shown and described in U.S. Pat. No. 3,842,428, and variations thereof are shown and described in U.S. Pat. No. 3,836,924 and U.S. Pat. No. 3,868,701. In these systems, the flash of light received by and reflected from the subject is metered by a light sensor disposed in a camera, rather than the incident light on the scene being metered, and the reflected light value is stored for the purpose of retaining this light value as the camera light sensors are moved or otherwise unavailable for metering during the actual taking of the photograph. Such systems are relatively complicated, require a preparatory flash and a main flash each time a photograph is taken and, significantly, suffer from many of the usual drawbacks of metering light reflected from the subject as distinguished from metering incident light on the subject.
2. Field of the Invention
The prevent invention relates to the field of electronic flash photography, and more particularly relates to an electronic incident flash memory system for enabling the incident light from an electronic flash unit to be more accurately controlled as well as to be preprogrammed.
3. Related Application
The subject matter of the present application relates to the subject matter of U.S. Application Ser. No. 55,538 filed concurrently herewith in the name of Ralph J. Gagnon and assigned to the assignee of the present application, and which discloses additional electronic circuits and systems useful in implementing the concepts of the present invention, the disclosure of which is incorporated herein by reference.